As the conventional gas component detection devices, there are available an infrared gas detector disclosed in Patent Document 1 and an infrared gas analyzer disclosed in Patent Document 2.
The conventional example disclosed in Patent Document 1 includes a housing into which a detection target gas (e.g., carbon monoxide) is introduced, a light source for irradiating infrared rays into the housing, and an infrared detector for detecting infrared rays existing within the housing. The light source is formed of a package-type light emitting diode which includes a light emitting diode chip (a bare element), a stem to which the bare element is mounted and an encapsulation material for encapsulating the bare element. The light source emits infrared rays as an electric power is supplied thereto from a lead terminal protruding from the stem. The infrared detector is formed of a package-type photo diode which includes a photo diode chip (a bare element), a stem to which the bare element is mounted and an encapsulation material for encapsulating the bare element. A detection signal is produced by a lead terminal protruding from the stem. The infrared detector is provided with a wavelength filter (a wavelength selecting filter) whose passband includes a wavelength region of infrared rays absorbed by a detection target gas. In this conventional example, a space having an ellipsoid shape is formed within the housing. The light emitting diode chip and the photo diode chip are positioned in two focal points of the ellipsoid. The conventional example disclosed in Patent Document 2 includes a box-shaped metal case, an elliptical reflection mirror arranged within the metal case, a light source and an optical receiver arranged to face the reflection surface of the elliptical reflection mirror, and a wavelength filter whose passband includes a wavelength region of infrared rays absorbed by a detection target gas, the wavelength filter provided on the light receiving surface of the optical receiver. A vent hole is formed in the metal case. A gas including a detection target gas (e.g., carbon dioxide) is introduced into the metal case through the vent hole. The concentration of the detection target gas existing within the housing or the metal case can be detected depending on the amount (level) of the infrared rays irradiated from the light source and received by the infrared detector or the optical receiver without being absorbed by the detection target gas.    [Patent Document 1] Japanese Patent Application Publication No. 2006-275980A    [Patent Document 2] Japanese Patent Application Publication No. H9-184803
The wavelength filter used in the conventional examples is usually formed of a band pass filter (an interference filter) including a dielectric multi-layer film. Therefore, the wavelength filter has a drawback in that the incidence angle dependence thereof is high. In other words, in a case of the conventional examples disclosed in Patent Document 1 or Patent document 2, there is a concern that, among the infrared rays having a wavelength belonging to a passband, most of the infrared rays, which are emitted from the light source and reflected by the inner surface of the ellipsoid or the reflection surface of the elliptical reflection mirror, may not pass through the wavelength filter. Further, when the amount of the infrared rays passing through the wavelength filter and reaching the infrared detector or the optical receiver decreases, the detection accuracy of gas components declines.
In view of the problems noted above, it is an object of the present invention to suppress the decrease in the amount of a received infrared ray attributable to the incidence angle dependence of a wavelength filter.